The invention relates to magnetic resonance angiography (MRA), and more particularly relates to subtraction MRA. In its most immediate sense, the invention relates to the suppression of non-blood tissue, e.g. fat and/or bone marrow, in ECG-triggered MRA of the leg and foot.
In certain MRA studies, it is important to suppress the contribution of fat, bone marrow and other non-blood tissues. This can be done by using subtraction MRA. In subtraction MRA, a mask image to which blood makes little or no contribution is subtracted from an angiographic image to which blood makes a major contribution. The resulting subtraction image emphasizes the image contribution of the blood and thereby makes it easier for the radiologist to visualize the patient's blood vessels.
It is known to form a subtraction image of a thin slice of interest by using a mask image acquired during the diastolic phase of the patient's heart cycle (when bloodflow is slow) and an angiographic image of the slice of interest acquired during the systolic phase of the patient's heart cycle (when bloodflow is rapid). However, it is difficult to obtain a high-quality subtraction image from a subtraction MRA study of a patient's leg.
One reason for this difficulty is that in peripheral regions of the body such as the legs, regurgitated arterial bloodflow (i.e. arterial bloodflow that is reversed in direction) exists. Such regurgitated arterial bloodflow may be unsaturated, and therefore may contribute to (and consequently contaminate) the mask image of the slice of interest. It is conventional to reduce or eliminate the signal from bloodflow in the direction opposite ordinary arterial bloodflow (e.g. venous bloodflow) by creating a saturation slab on the arterially downstream side of the slice of interest. However, in instances where bloodflow is regurgitated, such measures are not sufficient. This is because it is impractical to create a sharply-defined region in space that is fully saturated, while the surrounding region is fully unsaturated. Such an idealized fully saturated region is said to have a rectangular slice or saturation profile. Likewise, it is impractical to select the slice of interest in such a way as to create a rectangular slice profile. All practical saturation slabs and slices of interest are non-rectangular, i.e. all saturation slabs have transition zones of partial saturation at their upstream and downstream ends. Thus, while the signal from regurgitated arterial blood could be eliminated by means of a rectangular saturation profile precisely mated to a rectangular slice profile so as to eliminate any gap between them, this is impractical. To avoid saturating the blood within the slice of interest, it is common to space the slice to be imaged apart from the saturation slab and to thereby form a gap. Arterial blood within this gap will be partially saturated or unsaturated, and regurgitation of such blood may cause contamination of the mask image. A contaminated mask image will, as discussed below, produce a degraded subtraction image.
Another reason why it is difficult to produce a high quality subtraction MRA image of the leg has to do with the shortness of the relevant part of the patient's cardiac cycle. It is desirable to collect as much diastolic as systolic data within each heart cycle, so that angiographic and mask images of the same resolution may be acquired rapidly. The post-regurgitation duration of diastole does not last very long. Unless the patient has a very slow heart rate, or unless the amount of data collected during systole and diastole is unreasonably reduced, the post-regurgitation duration of diastole is not long enough to acquire a mask image of the same resolution as the angiographic (systolic) image. For this reason, it is presently impractical to begin acquiring the mask image after regurgitated bloodflow has ceased.
Hence, as explained above, in a subtraction MRA study of the leg, it is difficult to make sure that the mask image has sufficient resolution and is free from contamination. And, while it might be possible to improve the quality of the mask image by acquiring the mask and angiographic image data in separate acquisitions, this is also impractical. This would double the time required for the MR study. Furthermore, the patient would be more likely to move in the meantime, and such movement between acquisition of the images would degrade the diagnostic value of the resulting subtraction image.
It would therefore be advantageous to provide method and apparatus for producing a subtraction MR angiographic image of a slice of interest in a living patient in which diagnostically adequate and equivalent amounts of systolic and diastolic MR image data could be acquired during each cardiac cycle, and in which the diastolic MR image data would be uncontaminated by regurgitated arterial blood.
One object of the invention is to provide method and apparatus for producing a subtraction MR angiographic image of a slice of interest in a living patient, which would be suitable for use in peripheral MR studies, and particularly MR studies of the foot and lower leg.
Another object of the invention is to provide such method and apparatus in which equal amounts of MR mask image data and MR angiographic image data could be acquired during each cardiac cycle.
Still a further object of the invention is, in general, to improve on known methods and apparatus of this general type.
In accordance with the invention, a saturation slab is established immediately adjacent the slice to be imaged, and slightly overlapping the slice on its arterially downstream side. This prevents the existence of any gap between the slice and the saturation slab, and therefore insures that all regurgitated blood is saturated and does not contribute to the mask image. Significantly, this saturation slab is not established before each acquisition of MR image data during the cardiac cycle; this would partially saturate the systolic blood and reduce its contribution to the angiographic image. Likewise, this saturation slab is not established repeatedly during acquisition of diastolic MR image data; this would diminish the MR signal produced from the stationary tissues and would thereby degrade the mask image. Such a degraded mask image would produce an incomplete subtraction from the angiographic image acquired during systole and would consequently fail to suppress the image contribution from the stationary tissues.
Advantageously, and in accordance with the preferred embodiment, the patient's cardiac cycle is monitored and MR image data are acquired from the slice of interest during the systole phase of a particular cardiac cycle to form an angiographic image. Further in accordance with the preferred embodiment, the above-mentioned saturation slab is temporarily established at the end of the antegrade bloodflow during systole (i.e. before the beginning of regurgitated bloodflow during diastole) and diastolic MR image data are acquired immediately after such establishment has ceased, to form a mask image. Because this saturation slab is turned off while diastolic MR image data are acquired, the tissue within the slice of interest that is partially (or completely) saturated by the saturation slab recovers much of the magnetization previously lost to saturation. This increases the brightness of the stationary tissue in the mask image, and reduces the brightness of this tissue in the subtraction image. (As is conventional, the pulse sequences used to acquire MR image data may--and advantageously do--contain conventional RF saturation pulses. These have nothing to do with the referenced saturation slab.)
To achieve the best possible subtraction image, i.e. to establish an image in which the contribution of the stationary tissues has been eliminated as completely as possible, it is advantageous to match the intensity of the stationary tissues in the angiographic image to the intensity of the stationary tissues in the mask image. To do this, the saturation slab may additionally be temporarily established at an earlier point in the cardiac cycle, namely, just prior to acquisition of systolic MR image data. In this preferred embodiment, the stationary tissues are similarly partially saturated not only during diastole, but during systole as well.
Because the saturation slab eliminates the MR signal from regurgitated arterial blood, acquisition of diastolic MR image data for the mask image can begin immediately after the saturation slab has been established. This permits the acquisition of MR image data to take place over a sufficiently long time that the resolution of mask image can be equivalent to the resolution of the angiographic image.